Vehicles in which both the front and rear wheels are steered in many instances, utilize a computer or microprocessor controlled steering control arrangement. During a steering maneuver, the position and/or rate of movement of many of the components of the steering control arrangement are sensed electronically and electrical input signals directed to the microprocessor for controlling the position of both the front and rear wheels responsive to programmed conditions when requested by the vehicle operator.
It is generally believed that in the event of an electronic failure, manual controlling of both the front and rear wheels would be extremely difficult for the typical operator and that any back-up steering control should automatically center one set of wheels to the straight-ahead position with manual steering then utilizing the other set of wheels. One such steering system is disclosed in U.S. Pat. No. -A-4,770,264, in which one of the hydraulic actuators is spring biased to the straight-ahead position in the event of an electrical or hydraulic failure. One of the problems associated with that steering system is that the springs are necessarily compressed during each steering maneuver. Since such springs would necessarily have to be sufficiently large to center the associated wheels under the most adverse conditions, large expenditures of energy are consumed by the hydraulic system during every steering maneuver. Moreover, that system does not address the issue of providing a back-up steering control capable of steering the vehicle in the event of a hydraulic failure.
In view of the above, it would be desirable to provide a steering control system with a back-up steering control device which would automatically center one set of wheels to the straight-ahead condition in the event of an electronic failure and which would not utilize an excess amount of hydraulic energy during the normal steering maneuvers.
It would, moreover, be desirable to provide a back-up steering control circuit which provides an additional source of pressurized fluid for steering the vehicle in the event of a failure in both the electronic and hydraulic portions of the primary steering control.
The present invention is directed to overcoming one or more of the problems as set forth above.